1. Field of the Invention
This invention relates to the field of draw forming material, particularly to the art of forcing plastic deformation of metal tube stock in a die to produce formed tubes and hollow shafts.
2. Description of the Prior Art
Various cold-drawing techniques are known for producing tubing and hollow shafts having a wall thickness that is either constant throughout its length or that varies along its length. For example, U.S. Pat. Nos. 2,228,301, and 4,616,500 describe techniques for producing tubes having a relatively large diameter. The technique of the ""500 patent relates to a method that works well on large diameter tubing, wherein a removable plug of sufficient size is used to transmit large pulling or tension forces required to draw the tube through a die. However, on smaller diameter applications, such as when forming rear axles for front wheel drive vehicles and twist axles that both connect the rear wheels and serve as a stabilizer bar to control swing of a motor vehicle in turns, the conventional removable plug technique cannot be used effectively because of the relatively small initial size of the workpiece and the small inside diameter of the shaft being formed.
U.S. Pat. No. 3,572,080 describes a method for push pointing workpieces, a technique commonly used on a draw bench for cold working tubes and shafts from tube stock. It is best applied where the workpiece is approximately 6 feet or longer. A principal disadvantage of this technique is the resulting waste of a large amount of material. When drawing lengths of 6 feet or longer, the amount of waste material resulting from use of a pointed tube is insignificant compared to the loss of 5-8 inches on a workpiece that is 10-60 inches long. Therefore, when this technique is applied to an axle that is only 48 inches long, push-pointed tubes can result in waste of approximately 15 percent of the material required to produce them. The technique has a further disadvantage in that a pointed tube tends to collapse when a large tension force is applied to the reduced diameter transition section of the pointed tube.
Draw forming tubing to produce relatively short tubes and hollow shafts of relatively small diameter requires a technique that is compatible with applying relatively large drawing forces to the workpiece in order to produce a tube with a relatively thick wall at each end and a constant thin wall length in the region between the thick wall portions.
A broach puller is sometimes used to form the inside diameter of a tube with grooves such as rifling grooves on the inside diameter of gun barrels. Broach pullers are used to pull rods through a formed tube, restrained against displacement at a plate having an opening. Conventional broach pullers have a spring-loaded puller, which opens when the puller contacts a fixed restraining plate before pulling the broach bar through the tube. When the broach puller strikes the fixed plate, that contact compresses a helical compression spring located in the puller. The spring causes the jaws of the puller to open radially. As the puller is drawn away from the fixed plate, the compression force of the spring causes the radially positioned jaws to close and to engage a recess formed on the pull bar. This mechanism however will not release the pin except upon contact of the broach puller, usually at a fixed plate. In order to facilitate opening the jaws in a location other than that of the fixed plate, another technique is required to open and close the puller so that its jaws can be opened and closed in response to a command or a limit switch setting.
It is preferable in the process of cold drawing tubes having a variable wall thickness, particularly when the tubes have a small diameter and are relatively short, that a positive mechanical end connection, such as an end connection having engaged screw threads, be used to draw the tube through a reduction die and over a forming mandrel. The present invention provides a method for draw forming a thickened wall portion near an end of the tube, on whose inner surface screw threads are formed. An advantage of the invention is the provision of a reliable mechanical connection between the workpiece and equipment for drawing the tube, which connection requires a small space and provides easy engagement.
It is yet another advantage of the invention to minimize the amount of scrap or waste material that results from draw forming a workpiece to produce a tube or hollow shaft having a small diameter.
It is another advantage of the present invention to open and close the jaws of a gripper or puller on command regardless of its location.
In realizing these advantages, a method for producing a tube or hollow shaft from a workpiece having a tubular opening includes the steps of positioning near an end of the workpiece a reducing die for reducing the size of the outer surface of the workpiece and directing a portion of the workpiece into contact with an inner surface of the die; positioning within the die a mandrel having an outer surface and a plug, the mandrel and die defining a space between the inner surface and outer surface; advancing the die over the workpiece and the mandrel into the tubular opening along a portion of length of the workpiece, causing the workpiece to extrude into the space and to trap the plug behind the portion of the workpiece located in the space; and retracting the die and mandrel from the workpiece so that the plug is pulled along an inner surface of the portion of the workpiece located in the space. Then a screw thread is formed on the inner surface of the relatively thick wall portion of the workpiece.
An apparatus for realizing these advantages by producing an end connection on a workpiece having a tubular opening includes a reducing die having an inner surface formed with a transition section having a conical surface extending between a large diameter end and a small diameter end; and a substantially cylindrical surface having one end located at the small diameter end and extending axially therefrom; and a mandrel located at least partially within the die, having an outer surface formed with a second substantially cylindrical surface, and a plug located at an end of the second cylindrical surface, the plug having a larger diameter than that of the first cylindrical surface, the mandrel and die defining a space therebetween.